An investigation into the structural basis for informatio processing in the central nucleus of the inferior colliculus is proposed. It will combine anatomical techniques at both the light (LM) and electron microscopic (EM) levels to identify functionally distinct cell types. Neurons will be defined on the basis of their dendritic geometry and their patterns of synaptic organization by the LM and EM study of Golgi-impregnated neurons. These same features will be correlated with the destination of their axons by the backfilling of neurons with HRP. Neurons which are found to be structurally distinct will also be investigated by intracellular recording techniques to determine their responses to sound stimuli. Following intracellular injection and filling with HRP both the physiology and the morphology of the same neurons will be compared. Finally, the roles of different populations of presynaptic axons which arise from different cell types in the medulla will be investigated. In these experiments anterograde axonal transport of amino acids and LM/EM autoradiography will be utilized to label specific populations of axons and synaptic endings. Subjective and computer-assisted quantitative analyses of both labeling and axonal morphology will focus on the discrimination of different types of presynaptic endings. Thus, this study of the synaptic organization in the central nucleus of the inferior colliculus will emphasize the structure-function relationships which underlie the processing of auditory information in this important part of the midbrain.